1. Field of the Invention
This invention relates to an analog to digital converter.
2. Description of Related Art
Various techniques for converting analog signals into digital signals are known. They are discussed in, for example, "Intelligent Instrumentation--Microprocessor Applications in Measurement and Control" by Barney published by Prentice/Hall International (UK) Ltd.
Particularly for mass-produced items however, it is also desirable to be able to use low-cost components wherever possible as long as the quality of the converter remains acceptable for a given application.
The closed-loop technique of conversion is discontinuous. It takes a sample of an analog input, initiates a count and compares an analog version of the count value with the analog input. When parity is reached, the count stops and a digital output generated. Thus, the count starts at the same base value (usually zero) for each conversion which means the digital output is available only as quickly as the count can reach the correct value. This also means that the digital output is available after different conversion periods, depending on the analog value to be converted.
A form of continuous analog to digital conversion is possible using the simultaneous technique. However, it is rarely cost-effective as it requires 2.sup.n -1 comparators, which .sup.n is the number of bits in the digital output.